1.2- Diphenylethyl radical

The quenching of - -t by 1-methylpyrrole in nonpolar solvents results in the formation of a fluorescent exciplex, but no adduct formation (117). Irradiation in acetonitrile (anhydrous or aqueous) solution produces 1,2,3,4-tetraphenylbutane (65) in low yield. Formation of adducts 75 and 76 and 1,2-diphenylethane (64) is observed upon irradiation in acetic acid solution and attributed to protonation of t-lT followed by coupling of the 1,2-diphenylethyl radical and 1-methylpyrrole cation radical and deprotonation of the coupling product ... 

The reactions of - -t and 7 with secondary aliphatic amines are proposed to occur via formation of a nonfluorescent singlet ex-ciplex which yields a dialkylaminyl-l,2-diphenylethyl radical pair. The stereoselective formation of 69 indicates that radical pair combination is exclusively an in-cage process which competes effectively with rotation of the 1,2-diphenylcyclobutyl radical. The limiting quantum yields for the formation of 68 and 69 in nonpolar solvent are 0.14 and 0.16, respectively. Unlike the reaction of It with tertiary aliphatic amines, the quantum yield for the formation of 68 decreases with increasing solvent polarity (113). 

Arnold reported that the photoaddition of methanol and trifluoroethanol to 1,1-diphenylethene occurs in the photoreaction using methoxynaphthalenes as an electron donating sensitizer [35, 113]. The proposed mechanism for this reaction is shown in Scheme 34. The radical anion of the alkene is first produced by photoinduced electron transfer from the electron donating sensitizer to the alkene and it is protonated in a Markownikoff fashion to form the 1,1-diphenylethyl radical. The resulting radical is then oxidized by the radical cation of the electron donating sensitizer to generate the cation of the alkene. Finally, a nucleophilic attack of alcohol on this cation affords the alkoxylated product. 

Sulfur compounds in combination with peroxyl radical acceptors are often used for the efficient break of hydroperoxide [14]. The mechanism of action of these inhibitory mixtures can, however, be more complex, as demonstrated with reference to a pair of 2,6-diphenylphenol and distearyl dithiopropionate [15]. The combined addition of these compounds with concentrations of 0.05% and 0.3%, respectively, results in an extended inhibitory period during the oxidation of PP (up to 3000 h at 413 K). Sulfide (for instance, (3,(3 -diphenylethyl sulfide) or its products not only break down ROOH, but also reduce the phenoxyl radical. Sulfoxide formed in the reaction of the sulfide with ROOH can react with ArO. Thus, the ability of sulfides and their products to reduce phenoxyl radicals can contribute to their synergistic effect. 

The observation of decreased exciplex fluorescence intensity and increased adduct formation with increasing solvent polarity (Fig. 10) led to the proposal that adduct formation proceeds via initial one-electron transfer to yield a radical ion pair, followed by proton transfer to yield a 1,2-diphenylethyl and a-di-alkylaminoalkyl radical pair, which subsequently combines to yield 63, disproportionates or diffuses apart (114). Subsequent investigation of this reaction led to the proposal that proton transfer occurs only from the initially formed exciplex or contact radical ion pair prior to solvation to yield a solvent separated radical ion pair. The detailed mechanism for reaction of It with tertiary amines in acetonitrile solution is summarized in Fig. 11 (116c). 

Cyclization to form the ring-closed dihydrofluorenyl radical is a process that appears to be limited to diarylmethyl radicals that are substituted at the central carbon atom. Thus the excited triphenylmethyl, diphenylethyl, and diphenylcyclo-propylmethyl radicals all form ring-closed radicals whereas the parent diphenyl-methyl radical does not. The reason for this different behavior lies in the steric crowding produced by introducing a substituent at the central carbon atom. Whereas in the unsubstituted diphenylmethyl radical the angle between the phenyl... 

One-electron oxidation of an olefin, arene, or a bibenzyl group can lead to C—H or C—C bond cleavage to produce an allyl or benzyl radical [40, 41]. This area has been pioneered by Arnold [41], The PET reaction of 1,1,2,2-tetraphenylethane and methyl-3,3-diphenylethyl ether have been reported by Arnold and coworkers [41] to provide heterolytic C—C bond cleavage through an intermediate tetra-phenylethane cation-radical. The cation-radical intermediate fragments to di-phenylmethyl radical and diphenylmethyl carbocation. 

Solution-phase studies are more important preparatively. Two main mechanisms seem to operate in solution. The first is attack of the radical cation of a heteroaromatic donor on a tz nucleophile, as happens in the arylation reactions reported above. Other examples include photochemical reactions in which the heterocycle participates as a donor—for example the formation of 2- and 3-(l,2-diphenylethyl)-pyrroles (yield 44 and 10 %, respectively) from the irradiation of ( )-stilbene in the presence of pyrrole, a reaction which evidence implies is initiated by SET from pyrrole [88]. 2-(2, 2 -Diphenylethyl)furans are cleanly formed on irradiation of the corresponding furans in the presence of 1,1-diphenylethylene and an electron-accepting sensitizer [89]. Likewise, irradiation of naphthalene and benzothiophene in the presence of pyrrole results in electron transfer from the latter and leads eventually to pyrrolyldihydronaphthalene or benzothiophene, 44, respectively (Scheme 29) [90]. 

Peroxy radicals have also been observed by the one-electron reduction of cumyl, diphenylethyl, and triphenylmethyl hydroperoxides by cobaltous salts (Shchennikova et al., 1965). 

Values of 2k for these radicals will be given by an equation similar to (20) and as might be expected 2k depends on the nature of R because the t-RO produced by non-terminating interactions exhibit different susceptibilities to 3-scission. Somewhat more surprisingly values of 2k- also depend on the naXure of R. For instance the self-termination rate constant for 1,1-diphenylethyl-peroxy is over an order of magnitude larger than 2k- for t-BuOz. Since K and f do not depend on R it has been concluded that differences in k-j are due to differences in the rate constants for unimolecular decomposition of t-ROHR-t. 

Lindsay et at made a thorough study of the products of the self-reaction of 1-ethoxyethylperoxy and 1,2-diphenylethyl-peroxy radicals at ambient temperatures, radicals which would be expected to give the products outlined in Schemes III and IV. 

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